Temperature sensors are used in a variety of applications, where it is necessary either to compensate for variations in temperature or to provide control when a particular temperature is reached. It is well known that the base-emitter voltage V.sub.be of a transistor varies linearly with temperature, as does the difference between two such voltages .DELTA.V.sub.be. One known temperature sensor utilizing the variation in .DELTA.V.sub.be at different temperatures to provide an output indicative of the current temperature is described in "Micropower CMOS Temperature Sensor with Digital Output" by Anton Bakker and Johan H. Huijsing published in the IEEE Journal of Solid State Circuits, Vol. 31, No. 7, July 1996 at pages 933-937. The circuit basically consists of a pair of bipolar transistors having their bases in a fixed area ratio to each other. A known current is then passed through the two transistors to measure .DELTA.V.sub.be between them and thus get an indication of the temperature.
One problem with this type of circuit is that, although the transistors have their base areas nominally fixed at a particular ratio, manufacturing limitations mean that, in practice, the ratio is not the nominal one. Thus, the sensor requires calibration (trimming) at at least two different temperatures in order to set the gain and offset of the sensor correctly. This is time consuming and inconvenient.